The common slide rule, employing opposed ratio or logarithmic scales, has been in use for more than a hundred years. Even today, when micro-chips have replaced this device in most computing assignments, there still are applications where the slide rule has marketable advantages, and for this reason it will no doubt continue to be improved in a manner similar to that disclosed by the present invention.
Mechanically the slide rule has seen few changes or improvements over the decades. The original device, which carried a movable scale on a slidable member locked into a groove in a fixed member presenting an opposed scale, was improved by the introduction of circular or disc-shaped members coaxially mounted and which permitted elongation of the scales. Moving hairline cursors, slidable and rotatable, some with magnifying lenses, were added to facilitate reading. Discs were added. And windows were placed in some discs. Little else of real substance appears in the voluminous patent records. Purely mechanical improvements, in other words, have been few in number and in all probability will be even less evident in the future.
The hundreds of patentable improvements to the slide rule which are disclosed by a search of the records consist of the addition of new or modified scales to mechanical devices previously created by others. In most cases the patentable improvement has consisted of the adding of a single row of numbers to simplify the solving of a specific problem. The present invention is of this nature. A search has disclosed no prior slide rule which has printed thereon indicia of scale of chart or map and which in so doing eliminates the need to use dividers and latitude or mileage scale in solving time/speed/distance problems.